Coll de Serreïnes

Overview

The Coll de Serreïnes is one of those climbs that looks almost innocent on paper and turns savage the moment you start pedalling. At 6.5 km it is not long, but the road averages a punishing 10.81% the whole way up, dragging you from a start elevation of 747 m to a summit at 1445 m. That is 698 m of net vertical gain packed into a distance most riders would expect to cover in twenty minutes on flatter ground. Here, it will take a lot longer.

Rated Category 1, Serreïnes sits just below hors catégorie in difficulty. It earns that rating not through length but through relentlessness: a double-digit average gradient never lets you settle, and a maximum pitch of 21.07% waits somewhere in the middle to break anyone who started too hot.

How the Gradient Unfolds

The defining feature of Serreïnes is that there is no easy section. An average of 10.81% over 6.5 km means the climb offers almost nowhere to recover — the moments of relief are measured in metres, not kilometres. Where a gentler climb might give you a false flat to shake out your legs, here the road simply eases from severe to merely hard before tilting back up again.

The 21.07% maximum is the crux. A ramp nearly twice the average gradient forces a complete change of rhythm, and if it arrives when you are already deep into the 698 m of climbing, it can turn a controlled effort into a survival crawl. Reading the road early — spotting where the surface steepens — is the difference between spinning over the top and grinding to a halt.

Pacing Notes

On a climb this steep, pacing is everything. Ride the lower slopes at an effort you could hold for the full 6.5 km, not the effort that feels comfortable in the first minute. A useful anchor is your FTP: on Serreïnes, target a power just below threshold and treat the 21.07% ramp as the only place you are allowed to dip briefly into the red.

If you do not know your threshold, estimate it first with the FTP estimator, then work backwards to a sustainable climbing effort. With 698 m of vertical to absorb, going out too hard on the opening pitches is the single most common way riders blow up here.

Gearing

Do not underestimate the gearing you need. With a 10.81% average and pitches touching 21.07%, this is a climb that rewards the widest range you can fit. A compact 34-tooth chainring paired with a 34×32 low gear is the sensible minimum for most riders, and many will be far happier with a 34×34 to keep the cadence turning on the steepest ramps.

If your legs are not built for grinding, err toward the easier ratio — spinning a 34×34 up the 21.07% section will always beat muscling a 34×32 to a standstill.

Summary

Coll de Serreïnes is a short, ferocious Category 1 test: 6.5 km that never relents, climbing 698 m to a high summit. Get your pacing and gearing right and it is one of the most satisfying walls you can conquer.

Length: 6.5 km
Average gradient: 10.81%
Max gradient: 21.07%
Start: 747 m
Summit: 1445 m
Net gain: 698 m
Total ascent: 684 m
Category: 1